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Monitoring my solar panels from my PC

Paul Ockenden reveals how a Zigbee network helps him monitor the power generated by his solar panels

I recently had a roof-full of solar panels installed here at Ockenden Towers – but being the geek that I am, I paid extra to have something a bit different.

First, though, I’d like to explain something about solar panels – an issue the sales droid will probably fail to mention: the problem of "shading".

In a normal configuration solar panels are wired in series, so if at certain times of the day you get a little bit of shadow falling across one of them, it will reduce the output of the whole array; if one panel’s output is reduced 50% by shading, the output of all the others will be down 50% too. Even if you have a large, south-facing roof as I do, there’s a good chance that at certain times of the day you’ll get shading from a chimney, tree or perhaps a neighbour’s extension – this is the big secret of the solar PV (photovoltaics) industry.

The more I learn about ZigBee, the more I’m convinced that it will be important in the future

The solution is something called a micro-inverter: rather than have all the panels wired in series, delivering a massive – potentially lethal – DC voltage to a big "string" inverter, each solar panel has its own DC-to-AC micro-inverter strapped to the rear so that it operates independently of the other panels.

Actually, my own installation uses "duo" micro-inverters, where the output from two panels feeds into each box, so I need eight duo micro-inverters for my 16 panels, which is significantly more cost-effective than having one per panel. It means that a late-afternoon shadow from my chimney falling across one panel will take down only that panel pair rather than the whole array.

This is only one benefit of micro-inverters, another is their expected life span: they should last 25 years; a typical string inverter will need to be replaced every five to ten years. But it wasn’t their immunity from shading nor their longevity that grabbed my attention; it was the monitoring system. Each of the Enecsys micro-inverters I have contains a small 2.4GHz transmitter, and inside my house there’s a "gateway" box that receives signals from the micro-inverters and uploads them to a central monitoring system run by the company.

One of the reasons I was keen to use Enecsys micro-inverters in my solar PV installation is that the technology was developed at Cambridge University, where the company was spun out from the Department of Engineering. Enecsys was founded by the department’s professor and a couple of his PhD students, along with a bright spark from the adjacent business school. Although it’s now a global concern with international backers, its home-grown origin was an important factor in my buying decision.

ZigBee network

Communication between the micro-inverters and the gateway box is via a ZigBee network – an interesting, if curiously named, technology. It’s worth a small detour to examine what it is, what it does and how it does it.

ZigBee uses very low-power radios (0dB, or 1mW) to transmit data at the relatively slow rate of 250Kbits/sec, and has been designed from the ground up for use in applications such as switching, signalling and similar tasks that send small and infrequent bursts of data.

I reckon we’ll be seeing a lot more of ZigBee in the future because of its extreme power frugality: its single-chip radios are already low-powered, but what’s clever is the way in which they wake up, send their data, then go back to sleep again. Where a Bluetooth device might typically take around two or three seconds to wake up, a ZigBee system does it in around 30ms and it doesn’t need to be broadcasting all the time – it’s designed so that its transmitters can send data only when required without any constant background carrier signal. Its low power usage is important for a microgeneration project, where the power required to control and manage the system needs to be kept to a minimum.

The system employs the ISM band (2.4GHz), but because of its low power and the infrequent spurts of data, it works with minimal interference alongside other users of that band such as Wi-Fi and Bluetooth – in fact, I haven’t noticed any degradation of my wireless throughput.

ZigBee is capable of running in a mesh configuration but, as I understand it, the Enecsys system uses a simple, dynamic star configuration with all the nodes talking directly to the gateway.

It’s a fairly long communication chain: the panels feed their output to the micro-inverters; these then send monitoring data to the gateway; the gateway in turn uploads that data to Enecsys’ servers, where it’s made into pretty graphs, tables and dials, then downloaded to my PC or to an app on my iPhone or iPad.

Yet despite this lengthy datapath, I see the graphical representation of my panels updated pretty frequently: if the sun is suddenly obscured by a big cloud, both website and app generally show the effect within 30 seconds or so. This makes it easy to optimise the usage of the generated power, by switching on power-hungry loads such as dishwashers and washing machines only when the roof is generating lots of free electricity.

I’m really impressed by the system, and what started off simply as an exercise in getting a return on my investment, while also being green and saving the planet, has turned into a mini case study on using wireless data and online monitoring systems. The more I learn about ZigBee, the more I’m convinced that it will be important in the future, particularly in areas such as home automation, wireless alarm systems and wireless remote controls.